1. The Field of the Invention
The present invention relates to fiberglass ladders and to their method of manufacture. More particularly, it relates to the manufacture of fiberglass combination step and extension ladders which may be folded and unfolded from a stepladder configuration to a straight extension ladder configuration.
2. The Prior Art
Ladders are commonly used for a variety of applications and are of two general types: (1) a folding ladder, commonly called a stepladder, which is self-supporting, and (2) a straight extension ladder. Stepladders are typically used where it may be impossible to lean the ladder against a structure for support. On the other hand, an extension ladder is simply leaned against a wall or some other structure when standing or climbing on the ladder. Such ladders often include an extensible segment which can be used to telescopically extend the length of the ladder as desired.
Ladders which are constructed so that they may be used both as stepladders and as straight extension ladders are well-known in the art. See, e.g., U.S. Pat. Nos. 594,303; 1,100,823; 3,912,043; and 4,182,431. Typically, such ladders are constructed with hinges in the middle of the side rails. The hinges permit the ladder to be folded into a stepladder configuration or unfolded into a straight extension ladder configuration. As will be readily appreciated, combination step and extension ladders are very versatile and combine the desirable features of both types of ladders.
The combination step and extension ladders of the prior art are typically made of aluminum, steel, or other metal. While ladders constructed of such materials are suitable for most uses, the usefulness of a metal ladder near electrical currents is substantially limited. Because metal ladders are electrically conductive, the regulations of the Occupational Safety and Health Administration state that such ladders should not be used near live electrical wiring. For this and other reasons, the industry has long sought a suitable ladder, particularly a combination step and extension ladder, which can be made of a nonelectrically conductive material and which possesses the strength and stability necessary for use in construction and other industries.
Nonelectrically conductive materials used by those skilled in the art in the manufacture of a suitable ladder include various fiber/resin composites. To those skilled in the art, a "composite" is a material composed of fibers bonded in a resin matrix. Such composites are sometimes referred to (albeit imprecisely) by the generic term "fiberglass." (For convenience, the term "fiberglass" is sometimes used although it will be appreciated that other types of composites are equally applicable.) Composites, such as fiberglass, have been found to be excellent materials for the making of such ladders, not only because of the nonelectrically conductive property of composites but also because they are excellent energy absorbing materials (as illustrated by their use in helicopter rotors and polevault poles).
Unfortunately, fiberglass is an isentropic material; that is, its properties depend to a significant extent upon the orientation of the fibers within the fiberglass material. For example, fiber orientation affects such properties as the transverse, bearing, tensile, compression, and flexural strengths of the resultant fiberglass material, as well as the stiffness of the fiberglass. Accordingly, the fiber orientation can drastically affect the ability of a ladder constructed of fiberglass to withstand the pressures and stresses of normal usage.
While ladders made of composite materials are known in the art, such ladders have generally been made through a process known as "pultrusion." In general terms, the pultrusion process includes coating the fibers with a resinous material and then pulling the fibers through a heated die where the fibers harden into the desired shape; typically, the die is heated with microwaves.
Unfortunately, the pultrusion method results in the fibers being unidirectionally oriented within the fiberglass material. Although the fiberglass material has excellent longitudinal strength when the fibers are unidirectional, such a fiberglass material also has low flexural, transverse, and bearing strengths. Hence, when a ladder is constructed of such a material, the side rails are ofttimes incapable of withstanding the transverse bending and twisting forces exerted during typical use. Moreover, problems have been encountered in attaching the rungs to side rails made of unidirectional fibers such that the side rails are capable of supporting the rungs during usage.
In an attempt to overcome, to a limited extent, the problems encountered in making a ladder from unidirectional fiberglass, those skilled in the art have substantially increased the thickness of the fiberglass material and have combined a nonoriented fabric with the resinous coated fibers in order to impart sufficient strength to the fiberglass. However, such techniques, particularly the increasing of the thickness of the fiberglass, have resulted in a ladder which is much heavier and more cumbersome to use; such a ladder is also much more expensive to construct.
In the manufacture of any type of ladder, it is desirable to flare the lower portions of the side rails, i.e., bend the lower portion of the side rails outwardly to increase the distance between the side rails at the base of the ladder. This improves the stability of the ladder. However, it is difficult to form the side rails with such a flared portion using the pultrusion process of the prior art.
As will be appreciated, the problems encountered by the prior art with respect to a fiberglass ladder are greatly exaggerated when the ladder is extensible, such as in a combination step and extension ladder. In such ladders, there are both inner and outer side rails which are formed such that the inner side rails can be telescopically moved and extended within the outer side rails and can be locked into position. With such extensible ladders, two particular problems exist: (1) how to lock the inner side rails into position with respect to the outer side rails, and (2) how to attach the ladder rungs between the inner side rails and between the outer side rails such that they do not interfere with each other, but yet are fixedly attached to the respective side rails.
In typical prior art metal ladders, the inner side rails are locked into position by inserting a pin or other clamping device into a hole formed in the inner side rails. However, the inherent bearing strength weakness of fiberglass (particularly the unidirectional fiberglass of the pultrusion process) requires modification of that clamping device. This inherent weakness also makes the attachment of the rungs to the fiberglass side rails difficult. With fiberglass side rails, the rungs cannot be simply welded to the edges of the side rails.
It would, therefore, be an improvement in the art to provide a ladder made of a composite material which is capable of overcoming the inherent bearing strength weaknesses of the prior art fiberglass. It would also be advantageous to provide a fiberglass combination step and extension ladder which may be extended to increase the height of the ladder in both the straight extension ladder configuration and in the step ladder configuration such that the rungs attached to the respective side rails are securely mounted and are capable of withstanding the stresses and pressures of normal use.